Manufacture of mouldings from acrylonitrile copolymers



D. GORDON ET AL March 4, 1969 MANUFACTURE OF MOULDINGS FROM ACRYLONITRILE COPOLYMERS Filed June 14, 1965 United States Patent 25,044/64 U.S. Cl. 156--180 9 Claims 1m. (:1. D04h 3/08; 1329a 7/14 ABSTRACT OF THE DISCLOSURE Solid integral mouldings of polyacrylonitrile copolymer are prepared by wetting acrylonitrile copolymer fibres with a solution of an organic solvent admixed with an organic diluent, evaporating the wetted fibres content of solution until the fibres contain between 75% and 85 of organic solvent, and hot pressing the fibres to plasticise them and form a solid integral moulding.

The invention relates to the manufacture of mouldings from acrylonitrile copolymers. The term acrylonitrile copolymers is used by those skilled in this art to include co-polymers or ter-polymers consisting of at least 85% acrylonitrile with other monomers e.g. methylmethacrylate or vinyl acetate, either alone or to which have been added polymers compatible with them for example phenolic resins or Friedel-Crafts condensates. It is in this sense that the term acrylonitrile copolymer is used throughout the specification.

The present invention is concerned with producing acrylonitrile copolymer mouldings which possess good mechanical strength and which exhibit little deterioration in physical properties even under a wide range of climatic conditions. The invention is further concerned with producing mouldings in which the mechanical strength and dimensional stability are little affected in the presence of moisture. The invention also aims at producing acrylonitrile copolymer mouldings which have desirable electrical properties and which might be beneficially used in applications hitherto reserved for vulcanised fibre.

The drawing shows apparatus which may be employed in carrying out the process of the present invention.

According to the invention, acrylonitrile copolymer mouldings may be produced from acrylonitrile copolymer fibres by a process which comprises wetting the acrylonitrile copolymer fibres with an organic solvent for the fibres, reducing the organic solvent content of the fibres to a proportion between about 75 and 85% by weight of the unwetted fibres, heating the fibres to a temperature sufficient to plasticise at least a proportion of the fibres, applying moulding pressure to the heated fibres to bring them into intimate contact, and cooling the heated fibres, whereby the fibres are firmly bonded together to produce an integral moulding.

It is important that the solvent content of the fibres be reduced to 7585% by Weight of the unwetted fibre since higher proportions lead to the production of elastomeric mouldings which have little strength. Reduction of the solvent content much below about 75% results in the formation of a poorly bonded, unsatisfactory product. It is generally necessary, in order to achieve the desired proportion of solvent in the wetted fibre, to wet the fibres with solvent which has been diluted with a volatile organic diluent. The mass of fibres then take up both a volume of solvent and a volume of diluent instead of solvent only as in the case of undiluted solvent wetting, so that although the proportion of solvent in the former case is still higher than the required 75-85% range, it is rela- ICC tively easy to reduce this proportion of the required range by evaporation of the diluent and some solvent in an air stream. The proportion used is at least 25%, preferably about 50% by volume. There is no upper limit on the amount of diluent which might be used, but in practice little purpose is served in exceeding about by volume of diluent.

Residual solvent is generally present in the moulding when this is ejected from the moulding press and must be removed by conventional drying procedures in order to obtain a strong moulding.

Suitable organic solvents in which the fibres may be wetted include N,N-dimethylformamide (B.P. C.) ethyl carbonate (B.P. 238 C.) and dimethyl sulphoxide (B.P. C.) and the solvent is normally maintained at ambient temperatures during wetting.

The wetting process appears to be merely a physical wetting by the solvent of the surface regions of the fibres together with a small amount of penetration into the fibre. The application of heat to the wetted fibres, for example during the pre-pressing period in a heated moulding press, causes an interaction between the solvent and some re gions of the fibres which renders these regions plastic and allows the fibres to be bonded by pressure into a coherent mass. The plasticisation process is preferably arranged to be completed in the mould before moulding pressure is applied. It is of course important to keep the temperature of the wetted fibres in process stages prior to the plasticisation stage about 20 below the plasticisation temperature to avoid premature plasticisation.

The acryonitrile copolymer fibre used is conveniently in the form of a yarn although other forms, such as tow," consisting of parallel continuous filaments, or chopped tow, consisting of lengths of filament, or a woven fabric, a non-woven fabric, a knitted fabric, a felt, or a tape may be used. The fibres are conveniently any denier within the commercially available range of 2 /z9 denier.

A preferred procedure for moulding impregnated fibres in the form of a yarn consists in distributing the fibres evenly about a mould core and compressing in a mould between the platens of a moulding press. Where the fibre used is in the form of a yarn it is particularly convenient to wind the impregnated yarn evenly and continuously onto the former and so build up the required thickness of the moulding while at the same time ensuring a regular distribution of yarn throughout the mould. During the winding of the impregnated yarn onto the former it is preferable to maintain the tension in the yarn as low as possible so as to avoid premature expression of the solvent from the yarn.

In order to produce satisfactory mouldings it is necessary to use a moulding press in which the platens are heated to temperatures appropriate to the particular solvent used, generally within the range 60 to 110 C. Before applying moulding pressure to the impregnated fibres in the mould it is necessary to allow a short period for the fibres to attain a temperature near to that of the platens and for plasticisation to occur in the fibres. Failure to allow sufficient time before applying moulding pressure causes the expression of most of the solvent impregnating the fibres before plasticisation of these fibres has occurred and consequently results in poor bonding in the moulded product. The onset of plasticisation in the fibres may be detected by observing the behaviour of the force when closing the mould. Before plasticisation takes place, movement of the force is by stick-slip progression, indicating high friction between the fibres, whereas when plasticisation has occurred, the force moves steadily into the mould and meets a steadily increasing resistance from the plasticised fibres. Full moulding pressure should be applied when plasticisation occurs as delay generally results in the formation of an unduly swollen elastomeric moulding with an undesirably high solvent content. Excessive plasticisation is indicated by very free movement of the force and loss of polymer from the mould by extrusion.

A convenient moulding pressure is about 8000 lb. f./ sq. in., i.e. pounds force per square inch, although wide variations from this valve may be tolerated. The mould is preferably cooled to about 40 C. before ejecting the moulding in order that the moulding may be sufliciently rigid to be handled without damage.

When removed from the mould the moulding normally retains about 20 percent of entrained solvent which should be removed as completely as possible. Residual entrained solvent has a marked deleterious effect on the strength of the moulding as is illustrated by the table following Example 2. Entrained solvent may be reduced to less than 0.1 percent merely by allowing the moulding to stand in air at ambient temperatures. The removal of the solvent is assisted by passing an air stream over the moulding and a further increase in speed of removal may be obtained by warming the moulding to about 50 C. during the passage of the air stream. Heating at 50 C. may however cause a certain amount of discoloration of the surface of the moulding. Severe discoloration may occur if the moulding is heated above 50 C., and vacuum treatment (20 torr at 20 C.) causes distortion and cracking in the moulding when applied to a moulding still containing a substantial proportion of entrained solvent.

A typical semi-continuous process for the production of acrylonitrile copolymer fibre in the form of a yarn in accordance with the invention will now be described. The process is illustrated by the accompanying drawing.

Acrylonitrile copolymer fibres in the form of yarn is passed from a yarn spool through a solvent bath containing solvent and diluent in which it is thoroughly wetted. The wetted yarn is then passed through an evaporating tower where it meets a stream of air drawn through the tower by an exhaust fan. The evaporating tower reduces the solvent content of the wetted yarn to between 75% and 85% by weight of the original dry fibre weight and this yarn is then wound on a motor driven former. The former holding the yarn is then inserted into the heated moulding press and pressed to give a solid integral massive moulding of polyacrylonitrile.

Example 1 solvent is arranged to be of the order of seconds to r achieve sufficient wetting of the yarn with solvent. After passing through the solvent bath the wetted yarn is passed several times through an evaporating tower through which a. current of air is drawn by an exhaust fan. The current of air causes evaporation of any volatile organic diluent on the wetted yarn and also reduces the solvent content of the wetted yarn to about 7585% (by weight of unwetted yarn) by evaporation of some of the solvent. The wetted yarn is then wound evenly round a cylindrical former, care being taken to see that the tension on the yarn is as low as possible to prevent unintentional expression of solvent from the wetted yarn. The

desired thickness of wound yarn is built up on the former and the wound former is then placed in a moulding press, the former acting as the core of the mould. The platens of the press are pre-heated to a temperature sufiicient to cause some plasticisation throughout the thickness of the wound, wetted yarn, but insufficient to cause total loss of the fibrous nature of the wound yarn, and a light moulding pressure is applied for a period of about 5 minutes. Full moulding pressure is then applied for up to about 15 minutes. During the application of moulding pressure the bulk of thesolvent in the wound yarn is expressed and it is important that a mould clearance is left sufficient to allow for this expression. It is also important that the temperature of the yarn does not approach nearer than say about 20 C. below the temperature at which a significant degree of plasticisation of the fibres occurs at any stage of treatment before pre-heating in the moulding press.

After moulding is complete, the mould is cooled in order to cool the moulding suificiently to allow it to be handled during removal from the ress. At this stage the moulding may have retained up to about 20% by weight of solvent which may be removed by heating the moulding in an oven through which a slow stream of air passes to displace the liberated solvent. Alternatively, the mouldings may be dried under reduced pressure, or by other means.

A particular example of this semi-continuous process is as follows:

Example 2 A spool of acrylonitrile copolymer yarn (Courtelle yarn 6-denier, 6-inch staples spun to 4/4 woollen count) is passed through a bath containing a percent by volume solution of N,N'-dimethylformamide in dichloromethane maintained at ambient temperature. The impregnated yarn, containing about 200 percent by weight of this solution, is continuously withdrawn from the bath and passed continuously several times through a solvent evaporating tower which comprises a cylinder through which a strong current of air is passed. The passage through the tower removes substantially all the dichloromethane from the yarn and reduces the N,N'-dimethylfor-mamide content of the yarn to about percent by weight. The stripped yarn from the tower is wound evenly and continuously on to a former until the appropriate thickness of yarn has been built up. The wound former is then placed in a moulding press in which the platens are heated to 80 C. A light pressure (about 50 lb. f./sq. in. on the force) is applied to the mould for about 5 minutes while the yarn attains the temperature of the press. After this time, the pressure is raised to 8,000 lb. f./in. and maintained at this value for 15 minutes. The mould is then cooled to 40 C. and the work is ejected. The moulding thus formed contains about 20 percent by weight of N,N'-dimethylformamide and this is removed by heating the moulding in an oven at 40 C. and passing a slow stream of air over the moulding so as to displace the liberated vapour of N,N'-dimethylformamide.

The properties of typical mouldings produced in this way are shown by the following test results. The importance of the removal of residual solvent from the mouldings is clearly demonstrated.

Period of storage in air at 40 C; 10 months Property Unit more at room Nu 14 days 28 days temperature Hoop Strength (mean of 5) (standard dov.) Lb. f./ln. 33 Density at 20 C G./cm.' 1. 079 1. 155 1. 157 1. 158 Indentation at 20 C In. 10 7. 3 7. 0 7. 0 Heat distortion at C C 0. 1 Water absorption at 25 0...- 2. 1 Linear increase in dimensions 0. 6 Petrol absorption at 20 C 0. 1

The hoop strength is determined at 20 C. and 65 percent relative humidity on a Baldwintensile tester at 0.2 inch/min, using two semicircular yokes to rupture a test ring. The specific hoop strength is the hoop strength divided by the apparent density of the moulding.

The indentation is determined with a foot 0.225 x 0.02 inch, the long axis being transverse to the fibre direction in the test piece. An indenting pressure of 4,000 lb. f./in. is used.

The heat distortion is determined on a test ring of 1.35 inches inside and 1.65 inches outside diameter and 0.2 inch thick. The ring is cut radially and the upper limb so formed is clamped in a holder while the lower limb is loaded with a 50 g. weight. The assembly is heated in a glass-fronted oven at a rate of 50 C./hr. and the separation of the cut edges measured by a cathetometer.

The water absorption values are determined under equilibrium conditions with liquid water and the linear increase in dimensions when the test mouldings are saturated with water at 25 C.

A particular example of a batch process for the production of mouldings from acrylonitrile fibre in the form of a yarn is as follows:

A three-foot hank of acrylonitrile copolymer yarn (Courtelle yarn, 6-denier, 6-inch staple, spun to 4/4s woollen count), which contains about 30 yarns in crosssection, is treated with twice its weight of a 50 percent by volume solution of N,N'-dimethylformamide in dichloromethane solution. The hank of yarn is worked in the solution until the fibres are thoroughly wetted. The hank is then removed from the solution and subjected to an air stream until substantially all the dichloromethane has been removed and the N,N-dimethylformamide concentration of the impregnated yarn has been reduced to 80 percent by weight. The wetted yarn is charged spirally into an annular mould which is heated to 80 C. The moulding is thereafter pressed and dried as in the semicontinuous process (Example 1) hereinbefore described. Mouldings produced by this batch procedure tend to ex hibit uneven bonding due to the unequal distribution of yarn when it is charged into the annular mould.

Further particular examples of the procedure of mouldings from acrylonitrile copolymer yarn by either the continuous or the batch process described are as follows.

Example 3 100 parts of acrylonitrile copolymer yarn (Courtelle yarn, 6-denier, 6-inch staple spun to 4/4s woollen count) are wetted in 200 parts of a 50% by volume mixture of dimethyl sulphoxide solvent and dichloromethane diluent. The wetted yarn is subjected to an air stream to evaporate the dichloromethane diluent and to reduce the solvent content to about 80% by weight of unwetted yarn. The wetted yarn is wound on a former and then heated for about 5 minutes in a moulding press in which the platens are heated to about 90 C. to induce plasticisation. A moulding pressure of 8000 lb./sq. in. is then applied to the plasticised yarn for up to 15 minutes and the platen temperature is maintained at 90 C. during this period. The moulding is then cooled and ejected, and residual solvent is removed from it in the same way as in Examples 1 and 2.

Example 4 100 parts of acrylonitrile copolymer yarn (Orlon yarn, 2 /2 denier, spun to 4/ 4s woollen count) are wetted in 200 parts of a 50% by volume mixture of N,N'-dimethylformamide solvent and dichloromethane diluent. The solvent content of the wetted yarn is reduced to about 80% by weight of wetted yarn by an air stream. Moulding, pressing and drying are carried out as in the previous examples except that it is necessary to maintain the platens of the moulding press at 100 C. to achieve the appropriate degree of plasticisation of the fibres composing the yarn.

Example 5 100 parts of acrylonitrile copolymer tow (Acrilan tow, comprising 400 filaments in the hank having a filament denier of 2 /2) are immersed in 200 parts of a by volume mixture of N,N-dimet'hylformamide solvent and dichloromethane diluent. Evaporation in an air stream, moulding and removing the residual solvent are carried out as on the previous example except that it is necessary to maintain the platens of the moulding press at only C. to achieve satisfactory plasticisation of the fibres composing the tow.

The process provided by the present invention may be readily applied to acrylonitrile copolymer fibres in the form of a woven or a non-woven fabric. A typical process for the treatment of such fabric will now be described.

Example 6 A piece of acrylonitrile copolymer fabric is cut to a size suitable for charging into a mould and is thoroughly wetted with organic solvent by working the fabric in approximately twice its weight of organic solvent containing a volatile organic diluent. The wetted fabric is then suspended across an air stream to reduce the solvent content in the fabric between about 7585% and to remove the volatile organic diluent substantially completely. When the air stream has removed about one-third of the desired amount of diluent and solvent, the fabric is preferably inverted so as to present its other face to the air stream. This procedure ensures that the amount of evaporation from all parts of the fabric is reasonably uniform. The wetted fabric is then charged into the mould, the maximum permissible period between charging into the mould and the evaporation stage being about 15 minutes. The mould is pre-heated to a temperature sufiicient to cause plasticisation within the wetted fabric and a light pressure is applied to the fabric during this time. When plasticisation has occurred, as indicated by the ready compression achieved by force of the mould, full moulding pressure of about 8000 lbs/sq. in. is applied for up to about ten minutes. Moulding pressure is stopped when the resistance to the force reaches a steady value indicating good consolidation of the acrylonitrile copolymer fibres composing the fabric. The cooled moulding is then ejected. At this stage the moulding contains up to about 20% by weight 'of residual solvent which may be removed by subjecting the moulding to reduced pressure and/or temperatures up to about 50 C.

Particular examples of processes in accordance with the invention which produce acrylontrile copolymer mouldings from woven and non-woven fabrics will now be described.

Example 7 A batt of non-woven fabric (consisting of 2 /2 denier Courtelle fibres) having a weight of 5 ozs. per square yard is cut to size and thoroughly wetted in twice its weight of a 50% by volume mixture of N,N'-dimethylformamide and dichloromethane. The wetted batt is then subjected to an air stream, pressing and drying as described hereinbefore in Example 6. The platens of the press are maintained at about C. to produce the appropriate plasticisation before pressing.

The moulding produced has a hoop strength of about 5,500 pounds per square inch after the moulding has been dried at 50 C. for 30 days. The specific hook strength of the moulding is 7,200.

Example 8 A piece of woven fabric parts) consisting of an equal mixture of 3 denier and 4 /2 denier Courtelle fibres having a weight of 8 ozs. per square yard is thoroughly wetted in a 50% by volume mixture of N,N- dimethylformamide solvent and dichloromethane diluent (200 parts) and subjected to an air stream to reduce the solvent content of the wetted fabric to about 80% by weight of unwetted fabric. The fabric is then pressed and dried as described in Example 6 with the exception that the platens of the press are maintained at about 90 C. during the pro-pressing and pressing.

The moulding produced has .a hoop strength of about 5,300 pounds per square inch after the moulding has been dried at 40 C. for days. The specific hoop strength is 6,700.

Example 9 A sample of raised, woven acrylonitrile copolymer fabric, consisting of equal proportions of 3, 4 /2, 6 and 9 denier Courtelle (blanket fabric) having a weight of ozs. per square yard is treated under the conditions of Example 7.

The moulding produced has a hoop strength of 3000 pounds per square inch after 3 days drying at 50 C.

Example 10 Vulcanised Acrylonitrile copolbre ymcr moulding Density (g./cc.) 1. 17 1. 106 Moisture absorption at 75% relative humidity at 20 C. percent 4. 5 0. 06 Electric strength (kv./cm.) 60. 5 71.0

We claim: 1. A process for the production of a solid integral acrylonitrile copolymer moulding which comprises wetting acrylonitrile copolymer fibres with a solution consisting essentially of an organic solvent for said fibres selected from the group consisting of N,N'- dimethylformamide, dimethyl sulphoxide and ethylene carbonate admixed with an inert organic diluent having a boiling point below that of said organic solvent and selected from the group consisting of dichloromethane and diethyl ether so as to provide said fibres with at least 75 to 85% by weight of said organic solvent, based on the unwetted weight of said fibres, evaporating a portion of said solution from said fibres so as to reduce the organic solvent content of the wetted fibres to between about and by weight of the unwetted fibres,

heating the thus wetted fibres to a temperature between about 60 and 110 C. to plasticise at least a proportion of the fibres, applying moulding pressure to the heated fibres to bring them into intimate contact, and cooling the molded fibres to produce a solid integral moulding.

2. A process as in claim 1 in which the organic sol vent is N,N-dimethylformamide.

3. A process as in claim 1 in which the inert organic diluent is dichloromethane.

4. A process as in claim 1 in which the proportion of inert organic diluent to organic solvent in said solution is at least 1:3 [by volume.

5. A process as in claim 1 in which the solid integral moulding is finally subjected to sub-atmospheric pressure and temperature up to about 50 C. to remove residual solvent from the moulding.

6. A process as in claim 1 in which the organic solvent is dimethyl sulphoxide.

7. A process as in claim 1 in which the organic solvent is N,N'-dimethylformamide and the inert organic diluent is dichloromethane.

8. A process as in claim 1 in which the organic solvent is dimethylsulphoxide and the inert organic diluent is dichloromethanc.

9. A process as in claim 1 in which the fibres are wetted so as to provide them with up to by weight of said organic solvent based on the unwetted weight of said fibres.

References Cited UNITED STATES PATENTS 2,404,723 7/ 1946 Merner 260-326 3,053,609 9/1962 Miller 156-307 X 3,106,501 10/1963 Cobb et al. 156-180 3,144,025 8/ 1964 Erlich 161-150 X 3,148,101 9/1964 Allman et al. 156-167 3,152,919 10/1964 Biles et al. 117-138.8 3,167,448 1/1965 Hirshfeld 117-l38.8 3,236,587 2/1966 Genereux 8-130.1 3,365,354 1/1968 Britton 161-150 HAROLD ANSI-IER, Primary Examiner.

US. Cl. X.R. 

